For the purpose of more adequately describing a technological level for the present invention at present, all descriptions of the patents, patent applications, patent gazettes, scientific papers, and the like which are cited or specified in the present application will be incorporated herein by reference.
Interconnected circuits are very important for improvement in speeding up and packing density of a digital device and a communication device, and have a great effect on the electric characteristics of these devices. A printed circuit board including a multilayer board is cost-effective and convenient interconnect technology in which conductor layers which are mutually insulated by insulating layers are used for ground, power supply, and signals. This technology is particularly important in development and design of an interconnected circuit such as a planar transmission line including a micro-strip line, a strip line, a coplanar line, and a slot line as well as via transmission lines.
Conventionally, there has been proposed that ground via holes connected to ground plates are provided on a multilayer board to prevent an unnecessary propagation mode being produced by a high frequency signal leaked in a via structure through a insulating layer held between conductor plates. This prior art is disclosed in, for example, the Japanese Unexamined Patent Publication No. 9(1997)-321501.
In addition, the art that a coaxial line of predetermined impedance is formed to reduce a signal attenuation by forming the coaxial line of the predetermined impedance from a metal plate, an insulator, and a metal pin on a printed circuit board to permit the passage of a high-frequency signal therethrough is disclosed in, for example, the Japanese Unexamined Patent Publication No. 6(1994)-104601.
Furthermore, such an art that via holes for shielding electromagnetic waves are arranged on all sides so that the distance between each via hole is less than ½ of an effective wavelength to prevent the occurrence of resonance is disclosed in, for example, the Japanese Unexamined Patent Publication No. 2001-68596.
As described in the above patent gazettes, a typical example of an interconnection in a direction perpendicular to a printed circuit board includes a via hole structure. Simple via holes formed on the printed circuit board produce parasitic oscillation in an insulating layer comprising the printed circuit board to generate unnecessary electromagnetic radiation from the printed circuit board. For this reason, as described in Patent Document 3, the method using the ground via hole as the barrier is being carried out to avoid the effects of such unnecessary radiation.
However, while it is essential to accurately carry out impedance matching with respect to a via transmission line comprising one signal via hole and the barrier of the ground via holes in a case where the composite via transmission line is miniaturized, the fact is that a difficult problem that inevitably arises occurs.